Patent Literature 1 describes a method for manufacturing a composite film laminated with a light-transmitting thin film. Patent Literature 1 describes that, when ultrathin films are formed on both surfaces of a base film, light reflected from one of the ultrathin films formed on the back surface of the base film has an influence on measurement of the thickness of the other of the ultrathin films formed on the front surface of the base film. The method described in Patent Literature 1 is intended to reduce such an influence by mixing a light-absorbing material into the base film.
Patent Literature 2 describes a method for measuring a thickness of a thin film formed on a transparent substrate. Patent Literature 2 describes that, when a thin film formed on the front surface of the transparent substrate is irradiated with light and the thickness of the thin film is measured based on light reflected therefrom, light reflected from the back surface of the transparent substrate has an influence on the accuracy of the measurement. To solve this problem, in the measurement method described in Patent Literature 2, the thickness of the thin film is measured taking into account a back surface reflection coefficient contribution ratio γ being a ratio at which the light reflected from the back surface of the transparent substrate is detected.
Patent Literature 3 describes a method in which a multilayer thin film is irradiated with light and the thickness of the multilayer thin film is measured based on a spectrum of light reflected therefrom. In the measurement method described in Patent Literature 3, a fast Fourier transformation method is used to measure thicknesses of films on the front and back surfaces of a measurement target film in which the films are formed on both surfaces of a base material. The thicknesses of the films on the front and back surfaces are measured using reflected light in a low-transmissivity wavelength band and reflected light in a high-transmissivity wavelength band.
Patent Literature 4 describes a method for measuring a thickness of a film formed on a substrate. In the measurement method described in Patent Literature 4, so as to efficiently measure the thickness of the film on the substrate even if the roughness state of the surface of the substrate includes variations, the ratio (light-receiving ratio) of reflected light incident to a light receiving unit is calculated for each film thickness based on a relation between theoretical reflectance and light reception data in the case in which the substrate is mirror-surfaced, while changing the hypothetical value of the thickness of the film on the substrate. In addition, the light-receiving ratio and the theoretical reflectance are used to set model data of a reflected spectrum for the substrate having the hypothetical film thickness, and the model data is compared with the light reception data. Then, a film thickness corresponding to the model data with which the degree of agreement of the light reception data is the highest is identified as the thickness of the thin film.